The present invention relates to pressure sensitive switch mats. In particular, the present invention relates to pressure sensitive switch mats capable of interconnection with other such switch mats on all sides to eliminate non activation zones, or dead zones, therebetween.
Providing safety systems for dangerous machinery is commonly known. Examples of such safety systems include dual hand operated controls which forces an operator to keep both hands on the controls, presence sensing devices incorporating light beam and receivers, and singular electric switch mats connected to a machine control unit. With the utilization of larger and faster robotics and continuous operation machine tools, a need developed for the protection of areas too large to cover with individual electric switch mats and not protectable with light beam technology because of moving parts and equipment, irregular shapes of machinery, and other restrictions. Not only did a need develop, the federal government set safety standards which must be met in machine operating applications to ensure adequate worker safety. The Occupational Safety and Health Administration (OSHA) and the American National Standards Institute (ANSI) have set machine tool safety standards that dictate the criteria which must be met. The standard which has been set is that a safety mat device shall detect and respond to the presence, or absence, of the operator or others anywhere on its sensing surface. The sensing surface is defined as the area covered by a single mat or a plurality of mats in combination.
In an attempt to address the need for safety, electric switch mats came to be used in combination to protect areas not protectable by one mat. This practice, however, resulted in "dead zones", "non-activation zones", or "dead seams" where the mats abutted. The dead zones provide a narrow pathway through the protected area and do not comply with OSHA or ANSI standards. In an attempt to eliminate the dead zones, custom mats have been designed to fit individual applications. This approach, however, results in the need to engineer and produce individual mats for each specific application and results in a very high production cost and long lead times.
The use of multiple switch mats in combination precipitated the need for a safety control device between the mats and a machine control circuit. Multiple switch mats are electrically connected by switch mat wires and are connected to a machine control circuit that controls the machine about which the mats are placed. The safety control device serves as the joining point for the mat wires to consolidate the sensing signals into a single interface with which the machine control circuit is connected. The safety control device also serves as a means for monitoring the integrity of the wiring and the circuitry within the safety mat system. FIG. 1 illustrates a safety control 10 connected between an electronic switch mat 12 and a machine control circuit 14. The safety control 10 includes a power supply 16, a relay 18 and a status display 20. The relay 18 contains a relay coil 22 and a plurality of relay contacts 24. The relay contacts 24 include normally open contacts 25 and normally closed contacts 27. The electric switch mat 12 includes a top electrode plate 26 and a bottom electrode plate 28. The relay coil 22 of the safety control 10 is connected across the top and bottom electrode plates 26 and 28.
In operation, a current is applied through the top electrode plate 26. If there is no pressure on the electric switch mat 12, the current will flow through the top electrode plate 26 and into the relay coil 22. From the relay coil 22, current will flow back through the bottom electrode plate 28. When the relay coil 22 is energized, the relay contacts 24 move from their normally closed state 27 to a normally opened state 25. The machine control stop circuit 14 is connected across the normally opened contacts 25 which allows the machine being protected to operate when the relay coil 22 is energized. When pressure is applied to the electric switch mat 12, the top electrode plate is forced into contact with the bottom electrode plate 28 forming a short circuit. The current flowing in the top electrode plate is shorted into the bottom electrode plate, bypassing the relay coil 22. The relay contacts 24 return to their normally closed position 27 which opens the machine control stop circuit 14 and results in the machine being disabled.
The operating principle of the above system is sound but it does not meet ANSI machine safety standards for control reliability. The ANSI regulation for control reliability states that ". . . the device, system or interface shall be designed, constructed, and installed such that a single component failure within the device, interface or system shall not prevent normal stopping action from taking place but shall prevent a successive machine cycle." American National Standard, B 11.19-1990, .sctn.5.5.1. In the embodiment illustrated in FIG. 1, the failure of a single component could result in the unsafe operation of the machine. For example, if the welding of a single relay contact was to occur, the machine might still operate even with pressure applied to the electric switch mat 12. Thus, FIG. 1 does not meet ANSI control reliability standards.
There is a continuing need for a prefabricated, pressure sensitive switch mat that is capable of connecting with other such switch mats on all sides, and capable of covering irregular floor plans, thereby reducing ordering times, manufacturing costs, and engineering costs. There is also a need for a safety control device for controlling and monitoring switch mat integrity, machine circuit integrity and system integrity that meets set safety standards.